The blanketing or padding (pressurization) of chemical process systems with an inert gas is a concept widely known and used in the chemical industry to displace undesirable contaminants located in a vapor space and to prevent re-introduction of the contaminants into the vapor space. Padding with inert gas also provides an atmosphere in which the handling, processing, and reacting of chemicals can be performed safely and economically without fear of contamination, unwanted side reactions, or the development of potentially explosive mixtures.
As is commonly known, for combustion to take place in a chemical process, the three components of the "combustion triangle" must all be present. These components are oxygen, fuel, and an energy source. Combustion cannot occur if any one of these components is missing or otherwise present in an insufficient concentration.
For many chemical processes, a fuel component must be present in the system. The fuel itself usually forms an integral part of the process, and it is therefore highly impractical to eliminate this component of the combustion triangle.
An energy source, on the other hand, is usually an undesirable and ever present by-product of a chemical process. Therefore, eliminating an energy source from the process becomes a very difficult task, if not an impossible one. As an example, static electricity can be generated by a plurality of individual events which commonly occur in chemical processes. An energy source producing event can include the pneumatic conveying of finely ground solids through pipelines or chutes, agitation of single or multiple phase systems, movement of non-conductive liquids past a solid surface, high speed separation of two phase mixtures, liquids free-falling or splashing in air, aeration of single or multiple phase systems, etc. Clearly, with so many possible sources of static electricity, static electricity build-up is difficult to prevent. What is even more difficult to prevent is the eventual discharge of the generated electricity. As a result, the energy source component of the combustion triangle remains a highly impractical element to remove from a chemical process.
The final component, and most practical to control, is the concentration of oxygen in the system. By minimizing the oxygen concentration, one of the combustion triangle's key elements is thus eliminated, and the result is a safer environment in which the chemical process is allowed to occur with a less likely possibility of an unexpected explosion.
In addition to the reduction of oxygen, other chemical systems, in preparation for the handling of chemicals, may require the displacement of vapor space contaminants such as moisture or other vapor phase chemicals from a storage container. Two methods commonly used to accomplish elimination of vapor space contaminants along with oxygen reduction, are the continuous purge method involving high volume purging of the system with an inert gas, and the pad-depad method involving the pressurization and subsequent de-pressurization of the system with an inert gas. The pad-depad method is hereinafter referred to as the PDP method.
The continuous purge method can be ineffective in systems handling volatile chemicals where vapor pockets or stratification may occur. The flow path of the purging inert gas may not reach vapor pockets nor sufficiently disturb stratification. It cannot be assumed that the mere flooding of a system with a high volume purging inert gas will sufficiently dilute or displace all the vapor space undesirables in the system. The effectiveness of a continuous purge using inert gas is flow path and quantity dependent and can only be substantiated with costly oxygen analyzing systems. Subsequently, the net cost of performing high volume purges by inert gas to prepare systems for use of flammable chemicals is certain to be much higher than preparing the system using the PDP method.
The PDP method insures access to all spaces within a system, primarily because pressure permeates all open voids. Hence, attempts to reach suspect areas with an inert gas to dilute and subsequently displace the contaminant is guaranteed. The ability to dilute and replace a vapor space contaminant, such as oxygen with an inert gas such as nitrogen can be explained by the application of Boyle's-Charles' fundamental gas laws. In a confined space of a given volume, the pressure will increase when an equal volume of gas is introduced to the gas already present. By doubling the pressure in a confined space containing air (containing 21% oxygen) at atmospheric pressure, the actual volume of gas is essentially doubled at standard temperature and pressure. Since oxygen and nitrogen are infinitely soluble in each other, a doubling of the volume by the addition of nitrogen will decrease the net amount of oxygen per cubic foot of gas (air) at 1 atmosphere by 50%. Hence, by padding a system to 2 atmospheres, (approximately fifteen PSIG) with nitrogen and then depadding to one atmosphere (zero PSIG), the oxygen concentration in the mixture is reduced to 10.5%. Repetition of this procedure will continue to reduce the oxygen concentration by 50% each time, i.e., 5.25% for the second repetition and 2.125% for the third repetition.
The number of repetitions can be reduced by making each padding step an increase to three atmospheres (approximately thirty PSIG) and depadding to one atmosphere (zero PSIG). In this manner, the oxygen concentration in the gas (air) space is reduced by a factor of 2/3 each repetition, i.e. to 7% for the first repetition and 2.3% for the second repetition.
The PDP method is more reliable, inherently more predictable, and unquestionably more simple and economical than the continuous purge method. It also does not require the use of a highly skilled technician and is basically maintenance free, requiring only a source of inert gas as the motive force. Maintaining a constant positive pressure with appropriate pressure controls such as the PDP unit, using inert gas, after completion of the initial PDP sequence, will guarantee the presence of a relatively inert atmosphere. The need for expensive oxygen monitoring instrumentation can be reserved for extremely intense applications where the concentration of oxygen needs to be monitored for intermittent adjustments.
In the past, the decision to precisely control pressure in the low pressure range,( i.e. less than one PSIG) had been addressed by manifolding a common pressure regulator with a back pressure regulator. The back pressure regulator served to vent the system on high pressures and the pressure regulator provided make-up pressure on demand. This concept is simply not capable of precisely controlling the pressure by plus or minus 0.25 inches water column in the less than one PSIG range. Such a system exhibits very poor pressure control characteristics with fluctuations above and below 30% of the desired set point value.
A second prior art method, represented in FIG. 1, involves the use of a pad pressure regulator 100 and a low pressure relief device 110. An example of such a pad pressure regulator 100 can be found in U.S. Pat. No. 4,274,440 is hereinafter incorporated by reference. The pad pressure regulator 100 operates independently from the relief device 110 and concerns itself with the supply of gas in response to under-pressure conditions. The relief device 110, on the other hand, is designed to address the over-pressure situation relieving any excess pressure. These low pressure tank vent devices include a solid pallet, diaphragm pallet, liquid seal, or pilot operated design. The concern over vacuum is normally addressed in the same tank vent device. However, should a vacuum occur resulting from a failure of the pad pressure regulator 100, outside air will back flood the system, and in systems containing flammable solvents, this is especially undesirable as the resultant mixture could be explosive. Vacuum relief devices on systems containing flammable materials should, therefore, be equipped to back flood with an inert gas. Failure to recognize, understand, and address this issue with proper system design has, in the past, provided disastrous results.
A third prior art method involves the use of multiple pneumatic or electro-pneumatic control valves operating on a split range signal from an analog controller, as depicted in FIG. 2. A very sensitive pressure sensor-transmitter (PT) 210 transmits the process pressure to an electronic-type controller (PRC) 220 which sends a split range signal to the pad control valve (PCV) 250 and the depad control valve (PCV) 260 via signal lines 230 and 230', respectively. Signal lines 230 and 230' are the results of a split in a primary signal line 240. This approach can provide precision control in the less than one PSIG range, but at a significantly higher cost than the two previous methods. To install a system similar to the one illustrated in FIG. 2, one should expect to spend $12,000 to $20,000 for purchase and installation of all the components, and this estimate assumes that power and instrument air are in close proximity. This investment will also require the support of a competent instrument technician with programming experience for start-up and maintenance service. In addition, this system cannot operate independently from a source of power. Therefore, electrical or battery power becomes necessary.